Chapter 17--Star Stuff

Home , Black dwarf, Carbon star, Flare star, Guest star (astronomy), Helium flash, Red supergiant star

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17 Star Stuff

LEARNING GOALS

17.1 Lives in the Balance • What prevents carbon from fusing to heavier • What kind of pressure opposes the inward pull elements in low-mass stars? of gravity during most of a star’s life? 17.4 Life as a High-Mass Star • What basic stellar property determines how a star • In what ways do high-mass stars differ from low- will live and die? Why? mass stars? • How do we categorize stars by mass? • How do high-mass stars produce elements heavier 17.2 Star Birth than carbon? • Where are stars born? • What causes a supernova? • What is a protostar? • Do supernovae explode near Earth? • What are the “prebirth” stages of a star’s life? 17.5 The Lives of Close Binary Stars • What is a brown dwarf ? • Why are the life stories of close binary stars different 17.3 Life as a Low-Mass Star from those of single, isolated stars? • What are the major phases in the life of a • What is the Algol paradox? low-mass star? • How did past red giant stars contribute to the existence of life on Earth?

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I can hear the sizzle of newborn stars, A star can maintain its internal thermal pressure only and know anything of meaning, of the if it continually generates new thermal energy to replace fierce magic emerging here. I am witness the energy it radiates into space. This energy can come from to ﬂexible eternity, the evolving past, two sources: nuclear fusion of light elements into heavier and know I will live forever, as dust or ones and the process of gravitational contraction, which breath in the face of stars, in the converts gravitational potential energy into thermal energy shifting pattern of winds. [Section 15.1]. These energy-production processes operate only tem- Joy Harjo, Secrets from the Center of the World porarily, although in this case “temporarily” means millions or billions of years. In contrast, gravity acts eternally. Moreover, any time gravity succeeds in shrinking a star’s e inhale oxygen with every breath. core, the strength of gravity grows. (The force of gravity Iron-bearing hemoglobin carries this inside an object grows stronger if it either gains mass or shrinks in radius [Section 5.3].) Because a star cannot gen- oxygen through the bloodstream. W erate thermal energy forever, its ultimate fate depends on Chains of carbon and nitrogen form the backbone whether something other than thermal pressure manages of the proteins, fats, and carbohydrates in our cells. to halt the unceasing crush of gravity. The ﬁnal outcome of a star’s struggle between gravity Calcium strengthens our bones, while sodium and and pressure depends almost entirely on its birth mass. potassium ions moderate communications of the All stars are born from spinning clumps of gas, but new- nervous system. What does all this biology have to born stars can have masses ranging from less than 10% of the mass of our Sun to about 100 times that of our Sun. do with astronomy? The profound answer, recognized The most massive stars live fast and die young, proceeding only in the second half of the twentieth century, is from birth to explosive death in just a few million years. that life is based on elements created by stars. The lowest-mass stars, in contrast, consume hydrogen so slowly that they will continue to shine until the universe We’ve already discussed in general terms how is many times older than it is today. the elements in our bodies came to exist. Hydrogen Because of the wide range of stellar masses, we can and helium were produced in the Big Bang, and heav- simplify our discussion of stellar lives by dividing stars into three basic groups: ier elements were created later by stars and scat- ● tered into space by stellar explosions. There, in the Low-mass stars are stars born with less than about two times the mass of our Sun, or less than 2 solar spaces between the stars, these elements mixed with masses (2MSun) of material. interstellar gas and became incorporated into sub- ● Intermediate-mass stars have birth weights between sequent generations of stars. about 2 and 8 solar masses. In this chapter, we will discuss the origins of ● High-mass stars are those stars born with masses the elements in greater detail by delving into the lives greater than about 8 solar masses. of stars. As you read, keep in mind that no matter Both low-mass and intermediate-mass stars swell into how far removed the stars may seem from our every- red giants near the ends of their lives and ultimately become white dwarfs. High-mass stars also become red and day lives, they actually are connected to us in the large in their latter days, but their lives end much more most intimate way possible: Without the births, lives, violently. and deaths of stars, none of us would be here. We will focus most of our discussion in this chapter on the dramatic differences between the lives of low- and ypla om ce n . o c r o

t m

s high-mass stars. Because the life stages of intermediate-

a Stellar Evolution Tutorial, Lesson 1 mass stars are quite similar to the corresponding stages of high-mass stars until the very ends of their lives, we include 17.1 Lives in the Balance them in our discussion of high-mass stars. The story of a star’s life is in many ways the story of an Given the brevity of human history compared to the extended battle between two opposing forces: gravity and life of any star, you might wonder how we can know so much pressure. The most common type of pressure in stars is about stellar life cycles. As with any scientiﬁc inquiry, we thermal pressure—the familiar type of pressure that keeps study stellar lives by comparing theory and observation. On a balloon inﬂated and that increases when the tempera- the theoretical side, we use mathematical models based on ture or thermal energy increases. the known laws of physics to predict the life cycles of stars.

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On the observational side, we study stars of different mass itself. The clouds that form stars tend to be quite cold, typ- but the same age by looking in star clusters whose ages we ically only 10–30 K. (Recall that 0 K is absolute zero, and have determined by main-sequence turnoff [Section 16.6]. temperatures on Earth are around 300 K.) They also tend Occasionally, we even catch a star in its death throes. Theo- to be quite dense compared to the rest of the gas between retical predictions of the life cycles of stars agree quite well the stars, although they would qualify as a superb vacuum with these observations. by earthly standards. Like the galaxy as a whole, star-forming In the remainder of this chapter, we will examine in clouds are made almost entirely of hydrogen and helium. detail our modern understanding of the life stories of stars Star-forming clouds are sometimes called molecular and how they manufacture the variety of elements—the clouds,because their low temperatures allow hydrogen star stuff—that make our lives possible. atoms to pair up to form hydrogen molecules (H2). The relatively rare atoms of elements heavier than helium can also form molecules, such as carbon monoxide or water, or tiny, solid grains of dust. More important, the cold temperatures and high densities allow gravity to overcome thermal 17.2 Star Birth pressure more readily in molecular clouds than elsewhere Stars are born from clouds of interstellar gas (Figure 17.1) in interstellar space. If the thermal pressure in a molecular and return much of that gas to interstellar space when cloud is too weak to counteract the compressing force of they die. In Chapter 19, we will examine this star–gas–star gravity, then the cloud must undergo gravitational contrac- cycle in more detail. Here we will focus on star formation tion. Because molecular clouds are generally lumpy, gravity

Figure 17.1 A star-forming cloud of molecular hydrogen gas in the constellation Scorpius extends from VIS the upper-right corner of this photo through the center. The cloud appears dark because dust particles within it obscure the light radiated from more distant stars lying behind it. Blue-white blotches near the edges of the dark cloud are newly formed stars. They appear fuzzy because some of their light is reﬂecting off patchy gas in their vicinity. The region pictured here is about 50 light-years across.

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Figure 17.2 Fragmentation of a molecular cloud. Gravity attracts matter to the densest regions of a molecular cloud. If gravity can overcome thermal pressure in these dense regions, they collapse to form even denser knots of gaseous matter known as molecular cloud cores. The cloud thus fragments into a number of pieces, each of which will form one or more new stars.

pulls the molecular gas toward the densest lumps, known as molecular cloud cores. A cloud thus fragments into numerous pieces, each of which will form one or more new stars (Figure 17.2).

From Cloud to Protostar Betelgeuse Gravitational contraction within each shrinking fragment of a molecular cloud releases thermal energy. Early in the process of star formation, the gas quickly radiates away much of this energy, preventing thermal pressure from building high enough to resist gravity. Because the temperature of the cloud remains below 100 K, it glows in long-wavelength infrared light (Figure 17.3). The unopposed contraction initiated by gravity cannot continue indefinitely. As the cloud fragment contracts, Orion the resulting increase in density makes the escape of radia- Nebula tion increasingly difficult. Eventually, its central regions grow completely opaque to infrared radiation, trapping the thermal energy produced by gravitational contraction. Because thermal energy can no longer escape easily, both the thermal pressure and the gas temperature at the center of the contracting region rise dramatically. This rising pressure begins to fight back against the crush of gravity, Figure 17.3 This picture shows IR and the dense cloud fragment becomes a protostar— infrared radiation from star-forming the clump of gas that will become a new star. Meanwhile, regions in the constellation Orion. gaseous matter surrounding the protostar continues to The colors correspond to the rain down upon it, increasing its mass. temperature of the emitting gas: Red is cooler, and white is hotter. Star formation is most intense in the yellow-white regions, which have been heated to 60–100 K (which is still quite cold). The Disks and Jets Orion Nebula, home to many protostars, is the prominent yellow- white area near the bottom. Betelgeuse, a red supergiant, appears The rain of matter onto the protostar produces a proto- as a blue-white dot near the center. The region pictured here is stellar disk similar to the spinning disk from which the about 80 light-years across at the distance of the Orion Nebula. planets of our solar system formed [Section 9.2].A cloud

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a A cloud fragment necessarily has some angular momentum.

b This angular momentum causes it to spin faster and ﬂatten into a protostellar disk as it collapses.

c In the late stages of collapse, the central protostar has a strong protostellar wind and may ﬁre jets of high-speed gas outward along its rotation axis.

Figure 17.4 Artist’s conception of star birth.

fragment begins its collapse with a certain amount of an- THINK ABOUT IT gular momentum (the sum total of the angular momen- protostellar disk tum of each gas particle within it), which might at first be The term refers to any disk of material sur- unmeasurable. Contraction makes the angular momentum rounding a protostar. Do you expect all protostellar disks more obvious because conservation of angular momentum to eventually give birth to planets? If so, why? If not, what do you think might prevent planets from forming in some proto- [Section 5.2] demands that the cloud fragment spin faster and faster as it shrinks. Thermal pressure can halt the in- stellar disks? falling motion of the collapsing gas as it reaches the protostar but does nothing to stop its spinning motion. Much of The protostellar disk probably plays a large role in the infalling matter therefore settles into a spinning proto- eventually slowing the rotation of the protostar. The protostellar disk orbiting the protostar (Figure 17.4). These disks star’s rapid rotation generates a strong magnetic field. As sometimes coalesce into planetary systems like our solar the magnetic field lines sweep through the protostellar disk, system. We do not yet know how commonly this occurs. they transfer some of the angular momentum to outlying

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material, slowing the disk’s rotation [Section 9.3].The wider separations can arise when neighboring protostars strong magnetic field also helps generate a strong proto- form close enough together for their mutual gravity to stellar wind—an outward flow of particles similar to the keep them from drifting apart. solar wind [Section 15.2].The protostellar wind may carry Observations show that the late stages of a star’s for- additional angular momentum from the protostar to inter- mation can be surprisingly violent. Besides the strong protostellar space. stellar wind, many young stars also fire high-speed streams, Rotation is probably also responsible for the formation or jets,ofgas into interstellar space (Figure 17.5). No one of some binary star systems. Protostars that are unable to knows exactly how protostars generate these jets, but two rid themselves of enough angular momentum spin too fast high-speed streams generally flow out along the rotation to become stable single stars and tend to split in two. Each axis of the protostar, shooting in opposite directions. We of these two fragments can form a separate star. If the stars also sometimes see glowing blobs of material along the jets are particularly close together, the resulting pair is called a (named Herbig–Haro objects after their discoverers). These close binary system, in which two stars coexist in close prox- blobs appear to be collections of gas swept up as the jet imity and rapidly orbit each other. Binary systems with plows into the surrounding interstellar material. Together,

protostellar disk

protostar

1,000 AU jet

a Schematic illustration of protostellar b Photograph of a protostellar disk and jet. We are see- VIS disk–jet structure. ing the disk edge-on, as in (a). The disk’s top and bottom surfaces, illuminated by the protostar, are shown in green, and the jets emerging along the disk’s axis are shown in red. The dark central layers of the disk block our view of the protostar itself.

1,000 AU

c A wider-angle photograph of a jet emanating from a protostar (left) and ramming into surrounding VIS interstellar gas (right). Figure 17.5 Some protostars can be seen shooting jets of matter into interstellar space.

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winds and jets play an important role in clearing away the The length of time from the formation of a protostar cocoon of gas that surrounds a forming star, revealing the to the birth of a main-sequence star depends on the star’s protostar within. mass. Massive stars do everything faster. The contraction of a high-mass protostar into a main-sequence star may take From Protostar to the Main Sequence only a million years or less. A star like our Sun takes about 50 million years to go from the beginning of the protostellar A protostar looks starlike, but its interior is not yet hot stage to the main sequence. A very low-mass star of spectral enough for fusion. The central temperature of a protostar type M may spend more than a hundred million years as may be only a million degrees or so when its wind and a protostar. Thus, the most massive stars in a young star jets blow away the surrounding gas. To ignite fusion, the cluster may live and die before the smallest stars begin to fuse protostar needs to contract further to boost the central hydrogen in their cores and become main-sequence stars. temperature. We can summarize the transitions that occur during Paradoxically, radiation of thermal energy from the sur- star birth with a special type of H–R diagram. A standard face of a protostar is what enables its central temperature H–R diagram shows luminosities and surface temperatures to rise, because only half the thermal energy released by for many different stars [Section 16.5].This special H–R gravitational contraction is radiated away. The other half diagram shows part of a life track (also called an evolution- remains in the protostar’s interior, raising its temperature. ary track) for a single star in relation to the standard main If the protostar did not lose thermal energy from its sur- sequence. Each point along a star’s life track represents its face, it would not contract, and its central temperature surface temperature and luminosity at some moment dur- would remain fixed. Early in this period of contraction, ing its life. convection carries the protostar’s thermal energy to the Figure 17.6 shows a life track leading to the birth of a surface, as in the convection zone of the Sun [Section 15.4]. 1M star like our Sun. This prebirth period includes four However, as the interior of the protostar heats up, photons Sun distinct stages: can flow more easily, and radiative diffusion takes over from convection. Stage 1. When the protostar first assembles from a col- A protostar becomes a true star when its core tempera- lapsing cloud fragment, it is concealed within a shroud ture exceeds 10 million K, hot enough for hydrogen fusion of dusty molecular gas. It becomes visible after the proto- to operate efficiently by the proton–proton chain [Section 15.3]. stellar wind and jets disrupt this shroud. At this time, energy The ignition of fusion halts the protostar’s gravitational moves to the surface of the protostar primarily through contraction and marks what we consider the birth of a convection. At the end of this stage the photosphere’s tem- star. The new star’s interior structure stabilizes because the perature is about 3,000 K, placing it on the right side of energy produced in the center matches the amount radi- the H–R diagram, and its surface is many times larger than ated from its surface. The star is now a hydrogen-burning, that of the Sun, producing a luminosity somewhere be- main-sequence star [Section 16.5]. tween 10LSun and 100LSun.

Figure 17.6 The 6 life track of a 1MSun 10 star from protostar 3. Surface temperature 2. The protostar shrinks and heats as to main-sequence 10 5 rises when radiation gravitational potential energy is star. becomes the dominant converted into thermal energy. 10 4 mode of energy flow within the protostar. 10 3

2 1. A protostar assembles from 10 a collapsing cloud fragment. It is concealed beneath a 10 shroud of dusty gas. 1

0.1

luminosity (solar units) 2 4. The fusion rate increases 10 until it balances the energy radiated from the star’s surface. 10 3

10 4

10 5

30,000 10,000 6,000 3,000 surface temperature (Kelvin)

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Stage 2. The protostar’s surface temperature remains 6 near 3,000 K as long as convection remains the dominant 10 mechanism for transporting thermal energy to the surface. 10 5 60,000 years Gravitational contraction leads to a decrease in the proto- 15M 10 4 Sun star’s luminosity, because its radius becomes smaller while 150,000 years 9M its surface temperature stays nearly constant. Thus, the 10 3 Sun protostar’s life track drops almost straight downward on 3 mill 2 ion yea the H–R diagram. 10 rs 3MSun ears y 10 n o s i l r Stage 3. When energy transport within the protostar l i a

e m switches from convection to radiative diffusion [Section 15.4], 1 1MSun y 0 n

5 o i l the surface temperature begins to rise. This rise in surface Sun l i 0.1 m temperature brings a slight rise in luminosity, even though 50 luminosity (solar units) 0.5MSun 1 the protostar continues to contract. The life track there- 10 2

fore bends toward higher surface temperature and slightly 3 higher luminosity. During this stage, hydrogen nuclei begin 10 to fuse into helium nuclei, but the energy released is small 10 4 compared to the amount radiated away. 10 5 Stage 4. The core temperature and rate of fusion continue to increase gradually for a few tens of millions of years. 30,000 10,000 6,000 3,000 Finally, the rate of fusion becomes high enough to balance surface temperature (Kelvin) the rate at which radiative energy escapes from the surface. Figure 17.7 Life tracks from protostar to the main sequence for At this point, fusion becomes self-sustaining. The star settles stars of different masses. A high-mass protostar becomes a main- into its hydrogen-burning, main-sequence life. sequence star much more quickly than a lower-mass protostar, and its luminosity does not decline as much as it collapses. Protostars of different masses go through similar stages as they approach the main sequence. Figure 17.7 illustrates life tracks for several protostars of different masses. so furiously that gravity cannot contain their internal pressure. Such stars effectively blow themselves apart and drive THINK ABOUT IT their outer layers into space. Observations confirm the Explain in your own words what we mean by a life track for absence of stars much larger than 100MSun—if any stars a star. Why do we say that Figures 17.6 and 17.7 show only this massive existed nearby, they would be so luminous pre-main-sequence life tracks? In general terms, predict the that we would easily detect them. appearance on Figure 17.7 of pre-main-sequence life tracks On the other end of the scale, calculations show that the for a 25MSun star and for a 0.1MSun star. central temperature of a protostar with less than 0.08MSun never climbs above the 10 million K threshold needed Stellar Birth Weights for efficient hydrogen fusion. Instead, a strange effect called degeneracy pressure halts the gravitational contraction A single group of molecular clouds can contain thousands of the core before hydrogen burning can begin. of solar masses of gas, which is why stars generally are born The quantum mechanical origins of degeneracy pressure in clusters. We do not yet fully understand the processes are discussed in Chapter S4. A simple way to visualize the that govern the clumping and fragmentation of these clouds origin of degeneracy pressure is with an analogy to an audi- into protostars with a wide variety of masses. However, we torium in which the chairs represent all possible places that can observe the results. subatomic particles (electrons, in this case) can be located In a newly formed star cluster, stars with low masses and people represent the particles. Protostars with masses greatly outnumber stars with high masses. For every star above 0.08MSun are like auditoriums with many more avail- with a mass between 10 and 100 solar masses, there are typ- able chairs than people—the people (particles) can easily ically 10 stars with masses between 2 and 10 solar masses, squeeze into a smaller section of the auditorium. However, 50 stars with masses between 0.5 and 2 solar masses, and a the cores of protostars with masses below 0.08MSun are like few hundred stars with masses below 0.5 solar mass. Thus, auditoriums with so few chairs that the people (particles) although the Sun lies toward the middle of the overall fill nearly all of them. Because no extra chairs are available, range of stellar masses, most stars in a new star cluster are the people (particles) cannot squeeze into a smaller section less massive than the Sun. With the passing of time, the of the auditorium. This resistance to squeezing explains why balance tilts even more in favor of the low-mass stars as degeneracy pressure halts gravitational contraction. Keep the high-mass stars die away. in mind that the degeneracy pressure arises only because par- The masses of stars have limits. Theoretical models ticles have no place else to go. Thus, unlike thermal pressure, indicate that stars above about 100MSun generate power degeneracy pressure has nothing to do with temperature.

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Because degeneracy pressure halts the collapse of a few million years, dying before life could have arisen on a protostellar core with less than 0.08MSun before fusion Earth. Instead, the Sun has shone steadily for nearly 5 bil- becomes self-sustaining, the result is a “failed star” that lion years and will continue to do so for about 5 billion slowly radiates away its internal thermal energy, gradually more. In this section, we investigate the lives of low-mass cooling with time. Such objects, called brown dwarfs, stars like our Sun. occupy a fuzzy gap between what we call a planet and what we call a star. (Note that 0.08M is about 80 times the Sun Slow and Steady mass of Jupiter.) Because degeneracy pressure does not rise and fall with temperature, the gradual cooling of a brown Low-mass stars spend their main-sequence lives fusing hy- dwarf’s interior does not weaken its degeneracy pressure. drogen into helium in their cores slowly and steadily via the In the constant battle of any “star” to resist the crush of proton–proton chain [Section 15.3].As in the Sun, the energy gravity, brown dwarfs are winners, albeit dim ones. Their released by nuclear fusion in the core may take a million degeneracy pressure will not diminish with time, so gravity years to reach the surface, where it finally escapes into will never gain the upper hand. space as the star’s luminosity. The energy moves outward Brown dwarfs radiate primarily in the infrared and from the core through a combination of radiative diffusion actually look deep red or magenta in color rather than and convection [Section 15.4].Radiative diffusion transports brown. They are far dimmer than normal stars and there- energy through the random bounces of photons from one fore are extremely difficult to detect, even if they are quite electron to another, and convection transports energy by nearby. The first brown dwarf was discovered in 1995— the rising of hot plasma and the sinking of cool plasma. a 0.05MSun object (called Gliese 229B) in orbit around Radiative diffusion is more effective at transporting a much brighter star (Gliese 229A). Many more brown energy outward in the deeper, hotter plasma near a star’s dwarfs are now known. If the trend that makes small stars core.In higher layers, where the temperature is cooler, some far more common than massive stars continues to masses of the ions in the plasma retain electrons. These ions can below 0.08MSun,brown dwarfs might outnumber ordinary absorb photons and thereby tend to prevent photons from stars by a huge margin. continuing outward by radiative diffusion. When the energy welling up from fusion in the core reaches the point ypla om ce n . o c r o

t m

s at which radiative diffusion is inhibited, convection must

a Stellar Evolution Tutorial, Lesson 2 take over as the means of transporting energy outward. This point represents the beginning of a star’s convection 17.3 Life as a Low-Mass Star zone (Figure 17.8). In the grand hierarchy of stars, our Sun ranks as rather In the Sun, the temperature is cool enough to allow mediocre. We should be thankful for this mediocrity. If the convection in the outer one-third of its interior. More Sun had been a high-mass star, it would have lasted only massive stars have hotter interiors and hence shallower

high-mass star

1MSun star

very low mass star

Figure 17.8 Among main-sequence stars, convection zones extend deeper in lower-mass stars. High-mass stars have convective cores but no convection zones near their surfaces.

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convection zones. Lower-mass stars have cooler interiors 6 10 2 10 3 and deeper convection zones. In very low mass stars, the 10 Solar Radii Solar R adii 10 Solar Radii convection zone extends all the way down to the core. The 10 5 highest-mass stars have no convection zone at all near their 4 red giant surface, but they have a convective core because they pro- 10 1 Solar Radius duce energy so furiously. 10 3 The depth of the convection zone plays a major role 2 in determining whether a star has activity similar to that of 10 0.1 Solar Radius the sunspot cycle on our Sun [Section 15.5].The Sun’s activ- 10 subgiant ity arises from the twisting and stretching of its magnetic ﬁelds by convection and rotation. The most dramatically 1 Sun active stars are low-mass M stars that happen to have fast 0.1

rotation rates in addition to their deep convection zones. luminosity (solar units) 2 The churning interiors of these stars are in a constant state 10 of turmoil, twisting and knotting their magnetic ﬁeld lines. 10 3

When these field lines suddenly snap and reconfigure 4 themselves, releasing energy from the magnetic field, the 10 result can be a spectacular flare. For a few minutes or hours, 10 5 the flare can produce more radiation in X rays than the total amount of light coming from the star in infrared and 30,000 10,000 6,000 3,000 visible light. Life on a planet near one of these flare stars surface temperature (Kelvin)

might be quite difficult. Figure 17.9 The life track of a 1MSun star on an H–R dia- A low-mass star gradually consumes its core hydrogen, gram from the end of its main-sequence life until it becomes converting it into helium over a period of billions of years. a red giant. In the process, the declining number of independent particles in the core (four independent protons fuse into just one independent helium nucleus) causes the core to shrink “ash” left behind by hydrogen fusion—but the surround- and heat very gradually, pushing the luminosity of the maining layers still contain plenty of fresh hydrogen. Gravity sequence star slowly upward as it ages [Section 15.3].The shrinks both the inert (nonburning) helium core and the most dramatic changes occur when nuclear fusion exhausts surrounding shell of hydrogen, and the shell soon becomes the hydrogen in the star’s core. hot enough to sustain hydrogen fusion (Figure 17.10). The shell becomes so hot that this hydrogen shell burning Red Giant Stage proceeds at a higher rate than core hydrogen fusion did during the star’s main-sequence life. The energy released by hydrogen fusion during a star’s The result is that the star becomes more luminous than main-sequence life maintains the thermal pressure that ever before. Energy transport within the star is too slow to holds gravity at bay. When the core hydrogen is finally keep pace with this larger energy-generation rate. Because depleted, nuclear fusion ceases in the star’s core. With no much of this new thermal energy is trapped within the star, fusion to supply thermal energy, the core pressure can no thermal pressure builds up, pushing the surface of the star longer resist the crush of gravity, and the core begins to outward. What was once a fairly dim main-sequence star bal- shrink more rapidly. loons into a luminous red giant. While the red giant is large Surprisingly, the star’s outer layers expand outward on the outside, most of its mass is buried deep in a shrunken while the core is shrinking. On an H–R diagram, the star’s stellar core. life track moves almost horizontally to the right as the star The situation grows more extreme as long as the he- grows in size to become a subgiant (Figure 17.9). As the lium core remains inert. Recall that, in a main-sequence star expansion of the outer layers continues, the star’s lumi- like the Sun, a rise in the fusion rate causes the core to in- nosity begins to increase substantially, and the star slowly flate and cool until the fusion rate drops back down in a self- becomes a red giant.For a 1MSun star, this process takes correcting process called the solar thermostat [Section 15.3]. about a billion years, during which the star’s radius in- Thermal energy generated in the hydrogen-burning shell of creases about 100-fold and its luminosity grows by an even a red giant, however, cannot do anything to inflate the inert greater factor. (Like all phases of stellar lives, the process core that lies underneath. Instead, newly produced helium occurs faster for more massive stars and slower for less keeps adding to the mass of the helium core, amplifying its massive stars.) This process may at first seem paradoxical: gravitational pull and shrinking it further. The hydrogen- Why does the star grow bigger and more luminous at the burning shell shrinks along with the core, growing hotter same time that its core is shrinking? and denser. The fusion rate in the shell consequently rises, We can find the answer by considering the interior feeding even more helium ash to the core. The star is caught structure of the star. The core is now made of helium—the in a vicious circle.

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expanding photosphere

photosphere contracting inert helium core

hydrogen- hydrogen- burning core burning shell

Figure 17.10 After a star ends its main-sequence life, its inert helium core contracts while hydrogen shell burning begins. The high rate of fusion in the hydrogen shell forces the star’s upper layers to expand outward. main-sequence star expanding subgiant

The core and shell continue to shrink in size and the “alpha particles”) converts three helium nuclei into one shell grows in luminosity, while thermal pressure continues carbon nucleus: to push the star’s upper layers outward. This cycle breaks down only when the inert helium core reaches a temperature of about 100 million K, at which point helium nuclei can fuse together. (In a very low mass star, the inert helium energy core may never become hot enough to fuse helium. The core collapse will instead be halted by degeneracy pressure, 3 4He 1 12C ultimately leaving the star a helium white dwarf.) Through- out the expansion phase, a stellar wind carries away much Energy is released because the carbon-12 nucleus has a more matter than the solar wind does for our Sun, but at slightly lower mass than the three helium-4 nuclei, and the much slower speeds. lost mass becomes energy in accord with E mc2. The ignition of helium burning in low-mass stars has THINK ABOUT IT one subtlety. Theoretical models show that, in the inert Before you read on, briefly summarize why a star grows helium core of a low-mass star, the thermal pressure is too larger and brighter after it exhausts its core hydrogen. When low to counteract gravity. Instead, the models show that the does the growth of a red giant finally halt, and why? How pressure fighting against gravity is degeneracy pressure—the would a star’s red giant stage be different if the temperature same strange type of pressure that supports brown dwarfs. required for helium fusion were around 200 million K, rather Because degeneracy pressure does not increase with tem- than 100 million K? Why? perature, the onset of helium fusion heats the core rapidly without causing it to inflate. The rising temperature causes Helium Burning the helium fusion rate to rocket upward in what is called a helium flash. Recall that fusion occurs only when two nuclei come close The helium flash dumps enormous amounts of new enough together for the attractive strong force to overcome thermal energy into the core. In a matter of seconds, the electromagnetic repulsion [Section 15.3].Helium nuclei rapidly rising thermal pressure becomes the dominant pres- have two protons (and two neutrons) and hence a greater sure pushing back against gravity. The core is no longer positive charge than the single proton of a hydrogen nu- degenerate and begins to expand. This core expansion cleus. The greater charge means that helium nuclei repel pushes the hydrogen-burning shell outward, lowering its one another more strongly than hydrogen nuclei. Helium temperature and its burning rate. The result is that, even fusion therefore occurs only when nuclei slam into one though the star now has core helium fusion and hydrogen another at much higher speeds than those needed for hy- shell burning taking place simultaneously (Figure 17.11), drogen fusion. Therefore, helium fusion requires much the total energy production falls from its peak during the higher temperatures. red giant phase. The reduced total energy output of the star The helium fusion process (often called the “triple- reduces its luminosity, allowing its outer layers to contract alpha” reaction because helium nuclei are sometimes called from their peak size during the red giant phase. As the outer

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helium fusing into carbon in core

Figure 17.11 Core structure of a helium- burning star. Helium fusion causes the core hydrogen-burning and hydrogen-burning shell to expand and shell slightly cool, thereby reducing the overall energy generation rate in comparison to the rate during the red giant stage. The outer layers shrink back, so a helium-burning star is smaller than a red giant of the same mass.

layers contract, the star’s surface temperature also increases In a cluster of stars, those stars that are currently in somewhat. their helium-burning phase all have about the same lumi- Because the helium-burning star is now smaller and nosity but differ in surface temperature. Thus, on an H–R hotter than it was as a red giant, its life track on the H–R diagram for a cluster of stars, the helium-burning stars are diagram drops downward and to the left (Figure 17.12a). arranged along a horizontal branch (Figure 17.12b). This The helium cores of all low-mass stars fuse helium into car- cluster H–R diagram clearly shows the main-sequence bon at about the same rate, so these stars all have about the turnoff point. Stars that have recently left the main sequence same luminosity. However, the outer layers of these stars are subgiants on their way to becoming red giants. In the can have different masses depending on how much mass upper-right corner of the red giant region are stars that they lost through their stellar winds. Stars that lost more are almost ready for the helium ﬂash. The stars that have mass end up with smaller radii and higher surface tempera- already become helium-burning, horizontal-branch stars tures and hence are farther to the left on the H–R diagram. are slightly dimmer and hotter.

10 6 10 6 helium flash 5 5 stars nearly ready for 10 life track of star that lost 10 helium flash considerable mass 4 4 10 during red giant phase 10

10 3 10 3 red giants

10 2 10 2 horizontal branch

10 10 subgiants life track of star that lost less main-sequence 1 mass during red giant phase 1 turnoff point

0.1 Sun 0.1 luminosity (solar units) luminosity (solar units) 10 2 10 2

10 3 10 3

10 4 10 4

10 5 10 5

30,000 10,000 6,000 3,000 30,000 10,000 6,000 3,000 surface temperature (Kelvin) surface temperature (Kelvin) a After the helium ﬂash, a star’s surface shrinks and heats, so the b This H–R diagram plots the luminosity and surface temperature star’s life track moves downward and to the left on the H–R diagram. of individual stars in a cluster (i.e., it does not show life tracks). Helium core–burning stars occupy the horizontal branch of the diagram. A star’s position along the horizontal branch depends Figure 17.12 The onset of helium fusion. on how much mass it lost during the red giant phase.

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Last Gasps photospheres become especially carbon-rich in this way are called carbon stars. It is only a matter of time until a horizontal-branch star Carbon stars have cool, low-speed stellar winds, and fuses all its core helium into carbon. The core helium in a the temperature of the gas in these winds drops with dis- low-mass star will run out in about a hundred million years. tance from the stellar surface. At the point at which the tem- When the core helium is exhausted, fusion turns off, and the perature has dropped to 1,000–2,000 K, some of the gas core begins to shrink once again under the crush of gravity. atoms in these slow-moving winds begin to stick together The basic processes that changed the star from a main- in microscopic clusters, forming small, solid particles of sequence star into a red giant now resume, but this time dust. These dust particles continue their slow drift with helium fusion ignites in a shell around an inert carbon core. the stellar wind into interstellar space, where they become Meanwhile, the hydrogen shell still burns atop the helium interstellar dust grains.The process of particulate forma- layer. Both shells contract along with the inert core, driving tion is very similar to the formation of smoke particles in a their temperatures and fusion rates much higher. The lu- ﬁre. Thus, in a sense, carbon stars are the most voluminous minosity of this double-shell star grows greater than ever, polluters in the universe. However, this “carbon smog” is and its outer layers swell to an even huger size. On the H–R essential to life: Most of the carbon in your body (and in all diagram, the star’s life track once again turns upward (Fig- life on Earth) was manufactured in carbon stars and blown ure 17.13). Theoretical models show that helium burning into space by their stellar winds. inside such a star never reaches equilibrium but instead proceeds in a series of thermal pulses during which the fusion rate spikes upward every few thousand years. THINK ABOUT IT The furious burning in the helium and hydrogen shells Suppose the universe contained only low-mass stars. Would cannot last long—maybe a few million years or less for a elements heavier than carbon exist? Why or why not? 1MSun star. The star’s only hope of extending its life lies with the carbon core, but this is a false hope in the case of Before a low-mass star dies, it treats us to one last spec- low-mass stars. Carbon fusion is possible only at tempera- tacle. Through winds and other processes, the star ejects its tures above about 600 million K. Before the core of a low- outer layers into space. The result is a huge shell of gas ex- mass star ever reaches such a lofty temperature, degeneracy panding away from the inert, degenerate carbon core. The pressure halts its gravitational collapse. exposed core is still very hot and therefore emits intense For a low-mass star with a carbon core, the end is near. ultraviolet radiation that ionizes the gas in the expanding The huge size of the dying star means that it has a very shell. It now glows brightly as what we call a planetary weak grip on its outer layers. As the star’s luminosity and nebula.Despite this name, planetary nebulae have nothing radius keep rising, matter ﬂows from its surface at increas- to do with planets. The name comes from the fact that ingly high rates. Meanwhile, during each thermal pulse, nearby planetary nebulae look much like planets through strong convection dredges up carbon from the core, enrich- small telescopes, appearing as simple disks. Through a larger ing the surface of the star with carbon. Red giants whose telescope, more detail is visible. The famous Ring Nebula

Figure 17.13 The 6 double shellÐ inert carbon life track of a 1MSun 10 planetary nebula star from main- burning 5 helium-burning shell sequence star to 10 red giant hydrogen-burning shell white dwarf. Core 10 4 double shellÐ structure is shown red burning core at key stages. 10 3 helium- giant burning 10 2 star helium burning

10 subgiant hydrogen-burning shell 1 helium-burning Sun star core 0.1 luminosity (solar units) 10 2 white dwarf inert helium 10 3

10 4 hydrogen-burning shell

10 5 subgiant/ red giant core 30,000 10,000 6,000 3,000 surface temperature (Kelvin)

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in the constellation Lyra is a planetary nebula. Many other veers downward and to the right as the remaining ember planetary nebulae are also quite beautiful (Figure 17.14). cools and fades. You already know these naked, inert cores The glow of the planetary nebula fades as the exposed by the name white dwarfs [Section 16.5]. core cools and the ejected gas disperses into space. The neb- In the ongoing battle between gravity and a star’s ula will disappear within a million years, leaving behind internal pressure, white dwarfs are a sort of stalemate. As the cooling carbon core. On the H–R diagram, the life long as no mass is added to the white dwarf from some track now represents this “dead” core (see Figure 17.13). other source (such as a companion star in a binary system), At ﬁrst the life track heads to the left, because the core is neither the strength of gravity nor the strength of the de- initially quite hot and luminous. However, the life track soon generacy pressure that holds gravity at bay will ever change.

a Ring Nebula b Eskimo Nebula

c Spirograph Nebula d Hourglass Nebula

Figure 17.14 Planetary nebulae occur when low-mass stars in their ﬁnal death throes cast off their outer VIS layers of gas, as seen in these photos from the Hubble Space Telescope. The hot core that remains ionizes and energizes the richly complex envelope of gas surrounding it.

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Thus, a white dwarf is little more than a decaying corpse it is today (Figure 17.15). Earth’s surface temperature will that will cool for the indeﬁnite future, eventually disappear- exceed 1,000 K. Clearly, any surviving humans will need to ing from view as a black dwarf. have found a new home. Saturn’s moon Titan [Section 12.5] might not be a bad choice. Its surface temperature will have The Fate of Life on Earth risen from well below freezing today to about the present temperature of Earth. The Sun will shrink somewhat after The evolutionary stages of low-mass stars are immensely helium burning begins, providing a temporary lull in incin- important to Earth, because we orbit a low-mass star— eration while the Sun spends 100 million years as a helium- the Sun. The Sun will gradually brighten during its re- burning star. maining time as a main-sequence star, just as it has been Anyone who survives the Sun’s helium-burning phase brightening since its birth more than 4 billion years ago will need to prepare for one ﬁnal disaster. After exhausting [Section 15.3].The Sun’s past brightening has not threatened its core helium, the Sun will expand again during its last the long-term survival of life on Earth, because Earth’s million years. Its luminosity will soar to thousands of times climate self-regulates by adjusting the strength of the green- what it is today, and it will grow so large that solar promi- house effect (through the carbon dioxide cycle [Section 14.4]). nences might lap at Earth’s surface. Then it will eject its However, this climate regulation will eventually break outer layers as a planetary nebula that will engulf Jupiter down as the Sun warms. and Saturn and drift on past Pluto into interstellar space. We still do not understand Earth’s climate regulation If Earth is not destroyed, its charred surface will be cold well enough to be certain of when the warming Sun will and dark in the faint, fading light of the white dwarf that begin to overheat Earth. Some climate models predict that the Sun has become. From then on, Earth will be little the oceans will begin to evaporate about a billion years more than a chunk of rock circling the corpse of our once- from now, while other models suggest that our planet’s brilliant star. climate may remain stable much longer. All models agree ypla om ce n . o c r o

t m

that about 3–4 billion years from now, the Sun will have s

a Stellar Evolution Tutorial, Lesson 3 brightened enough to doom Earth to a runaway green- house effect like that on Venus [Section 10.6], causing the oceans to boil away. Temperatures on Earth will rise 17.4 Life as a High-Mass Star even more dramatically when the Sun ﬁnally exhausts Human life would be impossible without both low-mass its core supply of hydrogen, somewhere around the year stars and high-mass stars. The long lives of low-mass stars A.D. 5,000,000,000. allow evolution to proceed for billions of years, but only Things will only get worse as the Sun grows into a high-mass stars produce the full array of elements on which red giant over the next several hundred million years. Just life depends. Fusion of elements heavier than helium to before helium ﬂash, the Sun will be more than 100 times produce elements heavier than carbon requires extremely larger in radius and over 1,000 times more luminous than high temperatures in order to overcome the larger electro-

SPECIAL TOPIC Five Billion Years

The Sun’s demise in about 5 billion years might at first seem worri- we make that unfortunate choice, some species (including many some, but 5 billion years is a very long time. It is longer than Earth insects) are likely to survive. has yet existed, and human time scales pale by comparison. A Would another intelligent species ever emerge on Earth? There single human lifetime, if we take it to be about 100 years, is only is no way to know, but we can look to the past for guidance. Many 2 108,or two hundred-millionths, of 5 billion years. Because species of dinosaurs were biologically quite advanced, if not truly 2 108 of a human lifetime is about 1 minute, we can say that intelligent, when they were suddenly wiped out about 65 million a human lifetime compared to the life expectancy of the Sun is years ago. Some small rodentlike mammals survived, and here we roughly the same as 60 heart beats compared to a human lifetime. are 65 million years later. We therefore might guess that another What about human creations? The Egyptian pyramids have intelligent species could evolve some 65 million years after a human often been described as “eternal,”but they are slowly eroding due extinction. If these beings also destroyed themselves, another to wind, rain, air pollution, and the impact of tourists. All traces species could evolve 65 million years after that, and so on. of them will have vanished within a few hundred thousand years. Even at 65 million years per shot, Earth would have nearly 80 While a few hundred thousand years may seem like a long time, more chances for an intelligent species to evolve in 5 billion years the Sun’s remaining lifetime is more than 1,000 times longer. (5 billion 65 million 77). Perhaps one of those species will On a more somber note, we can gain perspective on 5 billion not destroy itself, and future generations might move on to other years by considering evolutionary time scales. During the past cen- star systems by the time the Sun finally dies. Perhaps this species tury, our species has acquired sufficient technology and power will be our own. to destroy human life totally, if we so choose. However, even if

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The Sun’s Luminosity a Solar luminosity. The Sun’s luminosity has ejection of gradually been rising over the past 4.6 bil- 10,000 planetary nebula lion years and will continue to rise slowly helium for another 5–6 billion years. Hydrogen shell flash burning will then drive a rapid increase in luminosity ending in a helium ﬂash when the 1,000 Sun’s luminosity reaches about 1,000LSun. The Sun will next enter a helium core– thermal pulses burning stage lasting about 100 million transition to years. After the core helium runs out, the 100 white dwarf luminosity will rise once again, peaking at a few thousand LSun with a series of thermal pulses. Note that the time scale is contraction helium expanded on the far right in order to show details of the ﬁnal stages. 10 of protostar leaves main core burning sequence luminosity (times present value) luminosity (times present 1 now

0510 12.112.2 12.3 12.3650 12.3655 the Sun’s age (billions of years)

The Sun’s Radius b Solar radius. The Sun’s radius increases ejection of with each rise in solar luminosity and declines planetary nebula with each fall. During the Sun’s ﬁrst red giant stage, it will reach a size roughly equal present radius of Earth’s orbit to Earth’s current orbit. (However, because the Sun’s mass and gravitational pull will be helium reduced by its strong wind during this stage, 100 flash Earth’s orbit will move to a slightly larger distance.)

thermal pulses

10 contraction of protostar leaves main helium sequence core burning radius (times present value) radius (times present

1 Figure 17.15 Evolution of the Sun. These now transition to graphs show calculations from models in white dwarf which the main-sequence lifetime of the Sun is 11 billion years, slightly greater than 051012.112.2 12.3 12.3650 12.3655 the more commonly quoted 10-billion-year the Sun’s age (billions of years) lifetime.

magnetic repulsion of more highly charged nuclei. Reach- will soon see, causes the star to self-destruct in the titanic ing such temperatures requires an extremely strong crush explosion we call a supernova. The fast-paced life and cata- of gravity on a star’s core—a crush that occurs only under clysmic death of a high-mass star—surely among the great the immense weight of the overlying gas in high-mass stars. dramas of the universe—are the topics of this section. The early stages of a high-mass star’s life are very similar to the early life stages of a low-mass star. However, Brief but Brilliant in the ﬁnal stages of their lives, the highest-mass stars proceed to fuse increasingly heavy elements until they have During the main-sequence life of a high-mass star, its strong exhausted all possible fusion sources. When fusion ceases, gravity compresses its hydrogen core to higher temperatures gravity drives the core to implode suddenly, which, as we than we ﬁnd in lower-mass stars. You already know that

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the rate of fusion via the proton–proton chain increases The escalated fusion rates in high-mass stars generate substantially at higher temperatures. The even hotter core remarkable amounts of power. Many more photons stream temperatures of high-mass stars enable protons to slam from the photospheres of high-mass stars than from the into carbon, oxygen, or nitrogen nuclei as well as into other Sun, and many more photons are bouncing around inside. protons. Although carbon, nitrogen, and oxygen make up Although photons have no mass, they act like particles and less than 2% of the material from which stars form in in- carry momentum [Section 5.1],which they can transfer to terstellar space, this 2% is more than enough to be useful anything they hit, imparting a very slight jolt. The combined in a stellar core. The carbon, nitrogen, and oxygen act as jolts from the huge number of photons streaming outward catalysts for hydrogen fusion, making it proceed at a far through a high-mass star apply a type of pressure called higher rate than would be possible by the proton–proton radiation pressure. chain alone. (A catalyst is something that aids the progress Radiation pressure can have dramatic effects on high- of a reaction without being consumed in the reaction.) mass stars. In the most massive stars, radiation pressure is The lives of high-mass stars are truly brief but brilliant. even more important than thermal pressure in keeping grav- The chain of reactions that leads to hydrogen fusion ity at bay. Near the photosphere, the radiation pressure in high-mass stars is called the CNO cycle; the letters CNO can drive strong, fast-moving stellar winds. The wind from stand for carbon, nitrogen, and oxygen, respectively. The a very massive star can expel as much as 105 solar mass six steps of the CNO cycle are shown in Figure 17.16. Just of gas per year at speeds greater than 1,000 km/s. This wind as in the proton–proton chain [Section 15.3],four hydro- would cross the United States in about 5 seconds and would gen nuclei go in while one helium-4 nucleus comes out. send a mass equivalent to that of our Sun hurtling into The amount of energy generated in each reaction cycle space in only 100,000 years. Such a wind cannot last long therefore is the same as in the proton–proton chain: It is because it would blow away all the mass of even a very equal to the difference in mass between the four hydrogen massive star in just a few million years. nuclei and the one helium nucleus multiplied by c2.The CNO cycle is simply another, faster way to accomplish Advanced Nuclear Burning hydrogen fusion. The exhaustion of core hydrogen in a high-mass star sets in motion the same processes that turn a low-mass star THINK ABOUT IT into a red giant, but the transformation proceeds much Did the very ﬁrst high-mass stars in the history of the universe more quickly. The star develops a hydrogen-burning shell, produce energy through the CNO cycle? Explain. and its outer layers begin to expand outward. At the same

Step 1 Step 2 Step 3 12C 13N 13N 13C 13C 14N Key: electron gamma ray neutrino neutron

positron proton

Total reaction 1H 1H

Step 6 Step 5 Step 4 12C 15N 15N 15O 15O 14N

1H 1 4He H

Figure 17.16 This diagram illustrates the six steps of the CNO cycle by which massive stars fuse hydrogen into helium. Note that the overall result is the same as that of the proton–proton chain: Four hydrogen nuclei fuse to make one helium nucleus. The carbon, nitrogen, and oxygen nuclei help the cycle proceed, but overall these nuclei are neither consumed nor created in the cycle.

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time, the core contracts, and this gravitational contraction The crush of gravity in a high-mass star is so over- releases energy that raises the core temperature until it be- whelming that degeneracy pressure never comes into play comes hot enough to fuse helium into carbon. However, in the collapsing carbon core. The gravitational contrac- there is no helium flash in stars of more than 2 solar masses. tion of the core continues, and the core temperature soon The high core temperatures induced by core contraction reaches the 600 million K required to fuse carbon into keep the thermal pressure high, preventing degeneracy heavier elements. Carbon fusion provides the core with a pressure from being a factor. Helium burning therefore new source of energy that restores the balance versus grav- ignites gradually, just as hydrogen burning did at the be- ity, but only temporarily. In the highest-mass stars, carbon ginning of the star’s main-sequence life. burning may last only a few hundred years. When the core A high-mass star fuses helium into carbon so rapidly carbon is depleted, the core again begins to collapse, shrink- that it is left with an inert carbon core after just a few hun- ing and heating until it can fuse a still-heavier element. dred thousand years or less. Once again, the absence of The star is engaged in the final phases of a desperate battle fusion leaves the core without a thermal energy source to against the ever-strengthening crush of gravity. Each suc- fight off the crush of gravity. The inert carbon core shrinks, cessive stage of core nuclear burning proceeds more rapidly the crush of gravity intensifies, and the core pressure, tem- than prior stages. perature, and density all rise. Meanwhile, a helium-burning The nuclear reactions in the star’s final stages of life shell forms between the inert core and the hydrogen- become quite complex, and many different reactions may burning shell. The star’s outer layers swell again. take place simultaneously (Figure 17.17). The simplest se- Up to this point, the life stories of intermediate- quence of fusion stages involves helium capture—the fusing mass stars (2–8MSun) and high-mass stars (>8MSun) are of helium nuclei into progressively heavier elements. (Some very similar, except that all stages proceed more rapidly helium nuclei still remain in the core, but not enough to in higher-mass stars. However, degeneracy pressure pre- continue helium fusion efficiently.) Helium capture can vents the cores of intermediate-mass stars from reaching fuse carbon into oxygen, oxygen into neon, neon into mag- the temperatures required to burn carbon or oxygen into nesium, and so on. anything much heavier. These stars eventually blow away At high enough temperatures, a star’s core plasma can their upper layers and finish their lives as white dwarfs. fuse heavy nuclei to one another. For example, fusing car- The rest of a high-mass star’s life, on the other hand, is un- bon to oxygen creates silicon, fusing two oxygen nuclei cre- like anything that a low- or intermediate-mass star ever ates sulfur, and fusing two silicon nuclei generates iron. experiences. Some of these heavy-element reactions release free neutrons,

Helium-capture reactions Figure 17.17 A few of the many nuclear reactions that occur in the ﬁnal 16O 20Ne 24Mg stages of a high-mass star’s life. Fusion 12 16 20 C (8p, 8n) O (10p, 10n) Ne (12p, 12n) of two silicon nuclei and some other processes that lead to iron actually ﬁrst produce nickel-56 (28 protons and 28 neutrons), but this decays rapidly to cobalt-56 (27 protons and 29 neutrons) and then to iron-56 (26 protons and 30 neutrons).

4He 4He 4He

Other reactions

12C 16O 28Si 28Si 31S 56Fe (14p, 14n) (16p, 15n) (26p, 30n)

16O 16O 28Si

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nonburning hydrogen hydrogen fusion

helium fusion

carbon fusion

oxygen fusion

neon fusion

magnesium fusion silicon fusion inert iron core Figure 17.18 The multiple layers of nuclear burning in the core of a high-mass star during the ﬁnal days of its life.

which may fuse with heavy nuclei to make still rarer ele- in a very important way: It is the one element from which ments. The star is forging the variety of elements that, it is not possible to generate any kind of nuclear energy. in our solar system at least, became the stuff of life. To understand why iron is unique, remember that only Each time the core depletes the elements it is fusing, two basic processes can release nuclear energy: fusion of light it shrinks and heats until it becomes hot enough for other elements into heavier ones, and fission of very heavy ele- fusion reactions. Meanwhile, a new type of shell burning ments into not-so-heavy ones. Recall that hydrogen fusion ignites between the core and the overlying shells of fusion. converts four protons (hydrogen nuclei) into a helium nu- Near the end, the star’s central region resembles the inside cleus that consists of two protons and two neutrons. Thus, of an onion, with layer upon layer of shells burning differ- the total number of nuclear particles (protons and neutrons ent elements (Figure 17.18). During the star’s final few combined) does not change. However, this fusion reaction days, iron begins to pile up in the silicon-burning core. generates energy (in accord with E mc2) because the mass Despite the dramatic events taking place in its interior, of the helium nucleus is less than the combined mass of the the high-mass star’s outer appearance changes only slowly. As each stage of core fusion ceases, the surrounding shell burning intensifies and further inflates the star’s outer 6 85MSun 10 40M layers. Each time the core flares up again, the outer layers Sun 5 may contract a bit. The result is that the star’s life track 10 25MSun zigzags across the top of the H–R diagram (Figure 17.19). 9MSun 10 4 In very massive stars, the core changes happen so quickly that the outer layers don’t have time to respond, and the 10 3

star progresses steadily toward becoming a red supergiant. 2 10 Betelgeuse, the upper-left shoulder star of Orion, is the best-known red supergiant star. Its radius is over 500 solar 10 radii, or more than twice the distance from the Sun to Earth. 1 Sun We have no way of knowing what stage of nuclear burning is now taking place in Betelgeuse’s core. Betelgeuse may have 0.1 luminosity (solar units) a few thousand years of nuclear burning still ahead, or we 10 2 may be seeing it as iron piles up in its core. If the latter is the case, then sometime in the next few days we will witness 10 3 one of the most dramatic events that ever occurs in the 10 4 universe. 10 5

Iron: Bad News for the Stellar Core 30,000 10,000 6,000 3,000 surface temperature (Kelvin) As a high-mass star develops an inert core of iron, the core continues shrinking and heating while iron continues to Figure 17.19 Life tracks on the H–R diagram from main-sequence pile up from nuclear burning in the surrounding shells. If star to red supergiant for selected high-mass stars. Labels on the tracks give the star’s mass at the beginning of its main-sequence iron were like the other elements in prior stages of nuclear life. Because of the strong wind from such a star, its mass can be burning, this core contraction would stop when iron fu- considerably smaller when it leaves the main sequence. (Based on sion ignited. However, iron is unique among the elements models from A. Maeder and G. Meynet.)

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four hydrogen nuclei that fused to create it—despite the Supernova fact that the number of nuclear particles is unchanged. The degeneracy pressure that briefly supports the inert iron In other words, fusing hydrogen into helium generates core arises because the laws of quantum mechanics pro- energy because helium has a lower mass per nuclear particle hibit electrons from getting too close together [Section S4.5]. than hydrogen. Similarly, fusing three helium-4 nuclei into Once gravity pushes the electrons past the quantum me- one carbon-12 nucleus generates energy because carbon chanical limit, however, they can no longer exist freely. The has a lower mass per nuclear particle than helium—which electrons disappear by combining with protons to form means that some mass disappears and becomes energy in neutrons, releasing neutrinos in the process (Figure 17.21). this fusion reaction. In fact, the decrease in mass per nuclear The electron degeneracy pressure suddenly vanishes, and particle from hydrogen to helium to carbon is part of a gravity has free rein. general trend shown in Figure 17.20. In a fraction of a second, an iron core with a mass The mass per nuclear particle tends to decrease as we comparable to that of our Sun and a size larger than that go from light elements to iron, which means that fusion of of Earth collapses into a ball of neutrons just a few kilo- light nuclei into heavier nuclei generates energy. This trend meters across. The collapse halts only because the neutrons reverses beyond iron: The mass per nuclear particle tends have a degeneracy pressure of their own. The entire core to increase as we look to still heavier elements. As a result, then resembles a giant atomic nucleus. If you recall that elements heavier than iron can generate nuclear energy only ordinary atoms are made almost entirely of empty space through fission into lighter elements. For example, uranium [Section 4.3] and that almost all their mass is in their nuclei, has a greater mass per nuclear particle than lead, so ura- you’ll realize that a giant atomic nucleus must have an nium fission (which ultimately leaves lead as a by-product) astoundingly high density. must convert some mass into energy. The gravitational collapse of the core releases an enor- Iron has the lowest mass per nuclear particle of all mous amount of energy—more than a hundred times what nuclei and therefore cannot release energy by either fusion the Sun will radiate over its entire 10-billion-year lifetime! or fission. Thus, once the matter in a stellar core turns to Where does this energy go? It drives the outer layers off iron, it can generate no further thermal energy or pressure. into space in a titanic explosion—a supernova.The ball of The iron core’s only hope of resisting the crush of gravity neutrons left behind is called a neutron star.In some cases, lies with degeneracy pressure, but iron keeps piling up in the remaining mass may be so large that gravity also over- the core until degeneracy pressure can no longer support it comes neutron degeneracy pressure, and the core continues either. What ensues is the ultimate nuclear-waste catastrophe. to collapse until it becomes a black hole [Section 18.4]. Theoretical models of supernovae successfully repro- THINK ABOUT IT duce the observed energy outputs of real supernovae, but How would the universe be different if hydrogen, rather than the precise mechanism of the explosion is not yet clear. Two iron, had the lowest mass per nuclear particle? Why? general processes could contribute to the explosion. In the first process, neutron degeneracy pressure halts the gravitational collapse, causing the core to rebound slightly and hydrogen ram into overlying material that is still falling inward. Until recently, most astronomers thought that this core-bounce

si process ejected the star’s outer layers. Current models of

n supernovae, however, suggest that the more important

l process involves the neutrinos formed when electrons and

s protons combine to make neutrons. Although these ghostly

e s particles rarely interact with anything [Section 15.3],so e

e many are produced when the core implodes that they drive

y a shock wave that propels the star’s upper layers into space. . helium The shock wave sends the star’s former surface zoom-

mass per nuclear particle carbon nergy. ing outward at a speed of 10,000 km/s, heating it so that eases e uranium Fission rel oxygen lead e iron Figure 17.21 During the ﬁnal, catastrophic collapse of a high-mass stellar core, electrons and protons 050100 150200 250 combine to form neutrons, accom- atomic mass (number of protons and neutrons) n panied by the release of neutrinos. Figure 17.20 Overall, the average mass per nuclear particle declines from hydrogen to iron and then increases. Selected nuclei are labeled to provide reference points. (This graph shows the most general trends only. A more detailed graph would show numerous p up-and-down bumps superimposed on the general trends.) neutrino

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it shines with dazzling brilliance. For about a week, a super- hydrogen nova blazes as powerfully as 10 billion Suns, rivaling the 1 helium luminosity of a moderate-size galaxy. The ejected gases slowly carbon cool and fade in brightness over the next several months, 2 10 oxygen but they continue to expand outward until they eventually neon mix with other gas in interstellar space. The scattered de- magnesium 4 bris from the supernova carries with it the variety of ele- 10 silicon argon sulfur calcium ments produced in the star’s nuclear furnace, as well as iron 6 additional elements created when some of the neutrons 10 nickel produced during the core collapse slam into other nuclei. Millions or billions of years later, this debris may be incor- 108 porated into a new generation of stars. nitrogen boron 10 The Origin of Elements 10

elative abundance (atoms per hydrogen atom) elative abundance (atoms per hydrogen beryllium Before we leave the subject of massive-star life cycles, it’s r 1012 lithium useful to consider the evidence that indicates we actually understand the origin of the elements. We cannot see inside 11020304050 stars, so we cannot directly observe elements being created atomic number (number of protons) in the ways we’ve discussed. However, the signature of Figure 17.22 This graph shows the observed relative abun- nuclear reactions in massive stars is written in the patterns dances of elements in the galaxy in comparison to the abundance of elemental abundances across the universe. of hydrogen. For example, the abundance of nitrogen is about For example, if massive stars really produce heavy ele- 10 4, which means that there are about 10 4 0.0001 times ments (that is, elements heavier than hydrogen and he- as many nitrogen atoms in the galaxy as hydrogen atoms. lium) and scatter these elements into space when they die, the total amount of these heavy elements in interstellar gas should gradually increase with time (because additional famous example concerns the Crab Nebula in the constel- massive stars have died). We should expect stars born lation Taurus. The Crab Nebula is a supernova remnant— recently to contain a greater proportion of heavy elements an expanding cloud of debris from a supernova explosion than stars born in the distant past because they formed (Figure 17.23). from interstellar gas that contained more heavy elements. A spinning neutron star lies at the center of the Crab Stellar spectra confirm this prediction: Older stars do Nebula, providing evidence that supernovae really do cre- indeed contain smaller amounts of heavy elements than ate neutron stars. Photographs taken years apart show that younger stars. For very old stars in globular clusters, ele- the nebula is growing larger at a rate of several thousand ments besides hydrogen and helium typically make up kilometers per second. Calculating backward from its pres- as little as 0.1% of the total mass. In contrast, about 2–3% ent size, we can trace the nebula’s birth to somewhere near of the mass of young stars that formed in the recent past A.D. 1100. Thanks to observations made by ancient astron- is in the form of heavy elements. omers, we can be even more precise. We gain even more confidence in our model of elemental creation when we compare the abundances of different Historical Observations The official history of the Sung elements. For example, because helium-capture reactions add Dynasty in China contains a record of a remarkable celes- two protons (and two neutrons) at a time, we expect nuclei tial event: with even numbers of protons to outnumber those with odd In the first year of the period Chih-ho, the fifth moon, numbers of protons that fall between them. Sure enough, the day chi-ch’ou, a guest star appeared approximately even-numbered nuclei such as carbon, oxygen, and neon several [degrees] southeast of Thien-kuan. After more are relatively abundant (Figure 17.22). Similarly, because ele- than a year it gradually became invisible. ments heavier than iron are made only by rare fusion reactions shortly before and during a supernova, we expect these This description of the sudden appearance and gradual elements to be extremely rare. Again, this prediction made dimming of a “guest star” matches what we expect for a by our model of nuclear creation is verified by observations. supernova, and the location “southeast of Thien-kuan” corresponds to the Crab Nebula’s location in Taurus. More- Supernova Observations over, the Chinese date described in the excerpt corresponds to July 4, 1054, telling us precisely when the Crab super- The study of supernovae owes a great debt to astronomers nova became visible on Earth. Descriptions of this par- of many different epochs and cultures. Careful scrutiny of ticular supernova also appear in Japanese astronomical the night skies allowed the ancients to identify several super- writings, in an Arabic medical textbook, and possibly in novae whose remains still adorn the heavens. The most Native American paintings in the southwestern United

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Before. The arrow points to the star observed to explode in 1987.

Figure 17.23 The Crab Nebula is the remnant of the supernova observed in A.D. 1054. This photo- VIS graph was taken with the Very Large Telescope at the European Southern Observatory in Chile.

States. Curiously, European records do not mention this supernova, even though it would have been clearly visible. Historical records of supernovae allow us to age-date the remnants we see today to determine the kinds of supernovae that produced them and to assess how frequently stars explode in our region of the Milky Way Galaxy. At least four supernovae have been observed during the past After. The supernova actually appeared as a bright point of light. It appears larger than a point in this photograph only because of thousand years, appearing as brilliant new stars for a few overexposure. months in the years 1006, 1054, 1572, and 1604. The supernova of 1006, the brightest of these four, Figure 17.24 Before-and-after VIS could be seen during the daytime and cast shadows at night. photos of the location of Super- nova 1987A. Supernovae may even have influenced human history. The Chinese were meticulous in recording their observations because they believed that celestial events foretold the future, and they may have acted in accord with such fortune- Supernova 1987A was the explosion of a star in the Large telling. The 1572 supernova was witnessed by Tycho Brahe Magellanic Cloud, a small galaxy that orbits the Milky and helped convince him and others that the heavens were Way and is visible only from southern latitudes. The Large not as perfect and unchanging as Aristotle had imagined Magellanic Cloud is about 150,000 light-years away, so [Section 3.4].Kepler saw the 1604 supernova at a time when the star really exploded some 150,000 years ago. he was struggling to make planetary orbits fit perfect cir- As the nearest supernova witnessed in four centuries, cles. Perhaps this “imperfection” of the heavens helped Supernova 1987A provided a unique opportunity to study push him to test elliptical orbits instead. a supernova and its debris in detail. Astronomers from all over the planet traveled to the Southern Hemisphere to Modern Observations: Supernova 1987A No super- observe it, and several orbiting spacecraft added observa- nova has been seen in our own galaxy since 1604, but today tions in many different wavelengths of light. astronomers routinely discover supernovae in other gal- Older photographs of the Large Magellanic Cloud axies. The nearest of these extragalactic supernovae, and allowed astronomers to determine precisely which star had the only one near enough to be visible to the naked eye, exploded (Figure 17.24). It turned out to be a blue star, not burst into view in 1987. Because it was the first supernova the red supergiant expected when core fusion has ceased. detected that year, it was given the name Supernova 1987A. The most likely explanation is that the star’s outer layers

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were unusually thin and warm near the end of its life, chang- close binary systems. The two stars in close binaries are ing its appearance from that of a red supergiant to a blue near enough to exert significant tidal forces on each other one. The surprising color of the preexplosion star demon- [Section 5.4].The gravity of each star attracts the near side strates that we still have much to learn about supernovae. of the other star more strongly than it attracts the far side. Reassuringly, most other theoretical predictions of stellar The stars therefore stretch into football-like shapes rather life cycles were well matched by observations of Super- than remaining spherical. In addition, the stars become nova 1987A. tidally locked so that they always show the same face to each One of the most remarkable findings from Super- other, much as the Moon always shows the same face to nova 1987A was a burst of neutrinos, recorded by neutrino Earth. detectors in Japan and Ohio. The neutrino data confirmed During the time that both stars are main-sequence that the explosion released most of its energy in the form stars, the tidal forces have little effect on their lives. How- of neutrinos, suggesting that we are correct in believing ever, when the more massive star (which exhausts its core that the stellar core undergoes sudden collapse to a ball of hydrogen sooner) begins to expand into a red giant, gas neutrons. The capture of neutrinos from Supernova 1987A from its outer layers can spill over onto its companion. has spurred scientific interest in building more purposeful This mass exchange occurs when the giant grows so large “neutrino telescopes.”Perhaps these neutrino telescopes that its tidally distorted outer layers succumb to the gravi- will open new fields of astronomical research in the com- tational attraction of the smaller companion star. The ing decades. companion then begins to gain mass at the expense of the giant. THINK ABOUT IT The solution to the Algol paradox should now be clear (Figure 17.25). The 0.8M subgiant used to be much When Betelgeuse explodes as a supernova, it will be more Sun more massive. As the more massive star, it was the first to than 10 times brighter than the full moon in our sky. If Betel- begin expanding into a red giant. As it expanded, however, geuse had exploded a few hundred or a few thousand years so much of its matter spilled over onto its companion that ago, do you think it could have had any effect on human his- it is now the less massive star. tory? How do you think our modern society would react if The future may hold even more interesting events for we saw Betelgeuse explode tomorrow? Algol. The 3.7MSun star is still gaining mass from its sub- ypla om ce n . giant companion. Thus, its life cycle is actually accelerating o c r o

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s a Stellar Evolution Tutorial, Lessons 1–3 as its increasing gravity raises its core hydrogen fusion rate. Millions of years from now, it will exhaust its hydrogen and 17.5 The Lives of Close begin to expand into a red giant itself. At that point, it can begin to transfer mass back to its companion. Even stranger Binary Stars things can happen in other mass-exchange systems, partic- For the most part, stars in binary systems proceed from ularly when one of the stars is a white dwarf or a neutron birth to death as if they were isolated and alone. The excep- star. That is a topic for the next chapter. tions are close binary stars. Algol, the “demon star” in the constellation Perseus, consists of two stars that orbit each Summary of Stellar Lives other closely: a 3.7MSun main-sequence star and a 0.8MSun subgiant. We have seen that the primary factor determining how a A moment’s thought reveals that something quite star lives its life is its mass. Low-mass stars live long lives strange is going on. The stars of a binary system are born and die in planetary nebulae, leaving behind white dwarfs. at the same time and therefore must both be the same age. High-mass stars live short lives and die in supernovae, We know that more massive stars live shorter lives, and leaving behind neutron stars and black holes. Both types therefore the more massive star must exhaust its core hy- of stars are crucial to life. Near the ends of their lives, low- drogen and become a subgiant before the less massive star mass stars can become carbon stars, which are the source does. How, then, can Algol’s less massive star be a subgiant of most of the carbon in our bodies. High-mass stars pro- while the more massive star is still burning hydrogen as duce the vast array of other chemical elements on which a main-sequence star? life depends. Mass exchange between close binary stars can This so-called Algol paradox reveals some of the com- complicate these basic patterns. Figure 17.26 summarizes plications in ordinary stellar life cycles that can arise in the life cycles of stars of different masses.

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Algol shortly after its birth. The higher-mass main- sequence star (left) evolved more quickly than its lower-mass companion (right).

Algol at onset of mass transfer. When the more massive star expanded into a red giant, it began losing some of its mass to its main-sequence companion.

Algol today. As a result of the mass transfer, the red giant has shrunk to a subgiant, and the main- sequence star on the right is now the more massive of the two stars.

Figure 17.25 Artist’s conception of the develop- ment of the Algol close binary system.

● THE BIG PICTURE The tug-of-war between gravity and pressure determines how stars behave from the time of their birth Putting Chapter 17 into Context in a cloud of molecular gas to their sometimes vio- In this chapter, we answered the question of the origin of lent death. elements that we ﬁrst discussed in Chapter 1. As you look ● Low-mass stars like our Sun live long lives and die back over this chapter, remember these “big picture” ideas: with the ejection of a planetary nebula, leaving be- ● Virtually all elements besides hydrogen and helium hind a white dwarf. were forged in the nuclear furnaces of stars. Carbon ● High-mass stars live fast and die young, exploding can be released from low-mass stars near the ends of dramatically as supernovae. their lives (carbon stars), and many other elements ● are released into space by massive stars in supernova Close binary stars can exchange mass, altering the explosions. usual course of stellar evolution.

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Protostars: A star system forms when a cloud of interstellar gas collapses under gravity. The central protostar is surrounded Blue main-sequence star: Star by a protostellar disk is fueled by hydrogen fusion in its in which planets may core. In high-mass stars, hydrogen eventually form. fusion proceeds by the series of reactions known as the CNO cycle.

Red supergiant: After core hydrogen is exhausted, the core shrinks and heats. Hydrogen shell burning begins around the inert helium core, causing the star to expand into a red supergiant.

Helium coreÐburning supergiant: Helium fusion begins when enough helium has collected in the core. The core Life of a 20MSun Star. then expands, slowing the fusion rate and allowing the star’s Main-sequence lifetime: 8 million years outer layers to shrink somewhat. Hydrogen shell burning Duration of later stages: 1 million years continues at a reduced rate.

Multiple shellÐburning supergiant: After core helium is exhausted, the core shrinks until carbon fusion begins, while helium and hydrogen continue to burn in shells surrounding the core. Late in its life, the star fuses heavier elements like carbon and oxygen in shells while iron collects in the inert core.

Neutron star: During the core collapse of the supernova, electrons combine with protons to make neutrons. The leftover core is therefore made almost entirely of neutrons. Supernova: Iron cannot provide fusion energy, so it accumulates in the core until degeneracy pressure can no longer support it. Then the core collapses, leading to the catastrophic explosion of the star. 2396_AWL_Bennett_Ch17 6/25/03 3:47 PM Page 569

Figure 17.26 Summary of stellar lives. The life stages of a Yellow main-sequence star: Star is fueled by high-mass star (on the left) and a low-mass star (on the right) hydrogen fusion in its core, which converts four are depicted in clockwise sequences beginning with the proto- hydrogen nuclei into one helium nucleus. In stellar stage in the upper left corner. (Stars not drawn to scale.) low-mass stars, hydrogen fusion proceeds by the series of reactions known as the protonÐproton chain.

Red giant star: After core hydrogen is exhausted, the core shrinks and heats. Hydrogen shell burning begins around the inert helium core, causing the star to expand into a red giant.

Helium coreÐburning star: Helium fusion, in which three helium nuclei fuse to form a single carbon nucleus, begins when enough helium has collected in the core. The core then expands, slowing the fusion rate and allowing the star’s outer layers to shrink somewhat. Hydrogen shell burning continues at a reduced rate.

Life of a 1MSun Star. Main-sequence lifetime: 10 billion years Duration of later stages: 1 billion years

Double shellÐburning red giant: After core helium is exhausted, the core again shrinks and heats. Helium shell burning begins around the inert carbon core and the star enters its second red giant phase. Hydrogen shell burning continues.

Planetary nebula: The dying star expels its outer layers in a planetary nebula, leaving behind the exposed inert core. White dwarf: The remaining white dwarf is made primarily of carbon and oxygen because the core never grew hot enough to fuse these elements into anything heavier. 2396_AWL_Bennett_Ch17 6/25/03 3:47 PM Page 570

SUMMARY OF KEY CONCEPTS

17.1 Lives in the Balance 17.3 Life as a Low-Mass Star • What kind of pressure opposes the inward pull of • What are the major phases in the life of a low-mass gravity during most of a star’s life? Thermal pressure, star? A low-mass star spends most of its life as a main- due to heat produced either by fusion or by gravita- sequence star, generating energy by fusing hydrogen tional contraction, opposes gravity during most of in its core. Then it becomes a red giant, with a hy- a star’s life. drogen shell burning around an inert helium core. • What basic stellar property determines how a star Next comes helium core burning, followed by double will live and die? Why? A star’s mass determines its shell burning of hydrogen and helium shells around fate, because it determines the star’s luminosity, an inert carbon core. When the star dies, it ejects its spectral type, and the kind of remnant it leaves a planetary nebula, leaving behind a white dwarf. behind. • How did past red giant stars contribute to the exis- • How do we categorize stars by mass? Low-mass stars tence of life on Earth? Red giants created and released much of the carbon that exists in the universe, in- are those born with mass less than about 2MSun. Intermediate-mass stars are those born with mass cluding the carbon that is the basis of organic molecules on Earth. between about 2 and 8MSun.High-mass stars are those born with mass greater than about 8MSun. • What prevents carbon from fusing to heavier elements in low-mass stars? Electron degeneracy pressure 17.2 Star Birth counteracts the crush of gravity, preventing the core • Where are stars born? Stars are born in cold, relatively of a low-mass star from ever getting hot enough dense molecular clouds—so named because they are for carbon fusion. cold enough for molecular hydrogen (H ) to form. 2 17.4 Life as a High-Mass Star • What is a protostar? A protostar is a compact clump of gas formed by gravitational contraction of a molec- • In what ways do high-mass stars differ from low-mass ular cloud fragment. A protostar in the early stages stars? High-mass stars live much shorter lives than of becoming a star is usually enshrouded in gas and low-mass stars. High-mass stars fuse hydrogen via dust. Because angular momentum must be conserved, the CNO cycle, while low-mass stars fuse hydrogen a contracting protostar is often surrounded by a via the proton–proton chain. High-mass stars die in protostellar disk circling its equator. Outﬂowing supernovae, while low-mass stars die in planetary matter from a protostar, in either a protostellar wind nebulae. Only high-mass stars can fuse elements or two oppositely directed jets, eventually clears heavier than carbon. A high-mass star may leave away the shroud of gas and dust. behind a neutron star or a black hole, while a low- mass star leaves behind a white dwarf. High-mass • What are the “prebirth” stages of a star’s life? (1) A stars are far less common than low-mass stars. protostar assembles from a cloud fragment and is bright in infrared light because gravitational contrac- • How do high-mass stars produce elements heavier than tion rapidly transforms potential energy into ther- carbon? Late in their lives, high-mass stars undergo mal energy. (2) Luminosity decreases as gravitational successive episodes of fusion of ever-heavier ele- contraction shrinks the protostar’s size. (3) Surface ments, producing elements as heavy as iron. Ele- temperature rises and luminosity levels off when ments heavier than iron are produced by these stars energy transport switches from convection to radia- when they die in supernovae. tive diffusion. (4) Core temperature and rate of • What causes a supernova? As a high-mass star ages, fusion gradually rise until energy production through carbon and heavier elements can fuse via helium cap- fusion balances the rate at which the protostar radi- ture and other processes to form ever-heavier ele- ates energy into space. At this point, the forming ments. Shells of increasingly heavy element fusion star becomes a main-sequence star. are created in the star’s core. However, because fu- • What is a brown dwarf ? A brown dwarf is a “star” that sion of iron uses up energy instead of releasing energy, never grows massive enough for efﬁcient nuclear an iron core cannot support the weight of the outer fusion in its core. Degeneracy pressure halts its grav- layers. The collapse of this core—which occurs in itational contraction before the core gets hot enough a fraction of a second—results in a supernova that for steady fusion. nearly obliterates the star (perhaps leaving a black hole or a neutron star).

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• Do supernovae explode near Earth? At least four • What is the Algol paradox? The star Algol is a binary supernovae have been observed in the Milky Way star in which the lower-mass star is in a more ad- Galaxy during the past thousand years: in 1006, 1054, vanced stage of life than the higher-mass star. This 1572, and 1604. Another supernova called Super- is a paradox because both stars must have been born nova 1987A was observed to explode in the Large at the same time and lower-mass stars should live Magellanic Cloud, a companion galaxy to the Milky longer, not shorter, lives. The explanation is that the Way, in 1987. lower-mass star was once the higher-mass star, but as it grew into a giant it transferred much of its mass 17.5 The Lives of Close Binary Stars to its companion. • Why are the life stories of close binary stars different from those of single, isolated stars? The transfer of mass from one star to its companion affects the life history (evolution) of both stars.

Sensible Statements? you think it is possible that it could harbor an advanced civilization. Explain your reasoning in one or two paragraphs. Decide whether each of the following statements is sensible and 9. A 10MSun main-sequence star. explain why it is or is not. 10. A ﬂare star. 1. The iron in my blood came from a star that blew up over 4 billion years ago. 11. A carbon star.

2. A protostellar cloud spins faster as it contracts, even though 12. A 1.5MSun red giant. its angular momentum stays the same. 13. A 1MSun horizontal branch star. 3. When helium fusion begins in the core of a low-mass star, 14. A red supergiant. the extra energy generated causes the star’s luminosity to rise. 15. Molecular Clouds. What is a molecular cloud? Briefly de- 4. Humanity will eventually have to find another planet to live scribe the process by which a protostar and protostellar disk on, because one day the Sun will blow up as a supernova. form from gas in a molecular cloud. 5. I sure am glad hydrogen has a higher mass per nuclear 16. Birth of a Close Binary. Under what conditions does a close particle than many other elements. If it had the lowest mass binary form? per nuclear particle, none of us would be here. 17. Protostellar Winds and Jets. Describe some of the activ- 6. I just discovered a 3.5MSun main-sequence star orbiting a ity seen in protostars, such as strong protostellar winds 2.5MSun red giant. I’ll bet that red giant was more massive and jets. than 3M when it was a main-sequence star. Sun 18. Life Tracks. What do we mean by a star’s life track on an 7. If the Sun had been born as a high-mass star some 4.6 bil- H–R diagram? How does an H–R diagram that shows life lion years ago, rather than as a low-mass star, the planet tracks differ from a standard H–R diagram? Jupiter would probably have Earth-like conditions today, while Earth would be hot like Venus. 19. Degeneracy Pressure. What is degeneracy pressure? How does it differ from thermal pressure? Explain why degener- 8. If you could look inside the Sun today, you’d find that its core acy pressure can support a stellar core against gravity even contains a much higher proportion of helium and a lower when the core becomes very cold. proportion of hydrogen than it did when the Sun was born. 20. Hydrogen Shell Burning. What happens to the core of a star when it exhausts its hydrogen supply? Why does hydrogen Problems shell burning begin around the inert core?

Homes to Civilization? We do not yet know how many stars have 21. Helium Fusion. Why does helium fusion require much Earth-like planets, nor do we know the likelihood that such plan- higher temperatures than hydrogen fusion? Brieﬂy describe ets might harbor advanced civilizations like our own. However, the overall reaction by which helium fuses into carbon. some stars can probably be ruled out as candidates for advanced 22. Planetary Nebulae. What is a planetary nebula? What hap- civilizations. For example, given that it took a few billion years for pens to the core of a star after a planetary nebula occurs? humans to evolve on Earth, it seems unlikely that advanced life would have had time to evolve around a star that is only a few 23. Fate of the Sun. Brieﬂy describe how the Sun will change, million years old. For each of the following stars, decide whether and how Earth will be affected by these changes, over the next several billion years.

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24. Advanced Nuclear Burning. Describe some of the nuclear Discussion Questions reactions that can occur in high-mass stars after they exhaust their core helium. Why does this continued nuclear 29. Connections to the Stars. In ancient times, many people be- burning occur in high-mass stars but not in low-mass stars? lieved that our lives were somehow influenced by the pat- 25. Formation of the Elements. Summarize some of the obser- terns of the stars in the sky, a belief that survives to this day vational evidence supporting our ideas about how the ele- in astrology. Modern science has not found any evidence ments formed and showing that supernovae really occur. to support this belief but instead has found that we have a connection to the stars on a much deeper level: In the words 26. Rare Elements. Lithium, beryllium, and boron are elements of Carl Sagan, we are “star stuff.”Discuss in some detail our with atomic numbers 3, 4, and 5, respectively. Despite their real connections to the stars as established by modern astron- being three of the five simplest elements, Figure 17.22 shows omy. Do you think these connections have any philosophi- that they are rare compared to many heavier elements. Sug- cal implications in terms of how we view our lives and our gest a reason for their rarity. (Hint: Consider the process civilization? Explain. by which helium fuses into carbon.) 30. Humanity in A.D. 5,000,000,000. Do you think it is likely 27. Future Skies. As a red giant, the Sun’s angular size in Earth’s that humanity will survive until the Sun begins to expand sky will be about 30°. What will sunset and sunrise be like? into a red giant 5 billion years from now? Why or why not? About how long will they take? Do you think the color of If the human race does survive, how do you think people the sky will be different from what it is today? Explain. in A.D. 5,000,000,000 will differ from people today? What 28. Research: Historical Supernovae. As discussed in the text, do you think they will do when faced with the impending historical accounts exist for supernovae in the years 1006, death of the Sun? Debate these questions, and see if you 1054, 1572, and 1604. Choose one of these supernovae and and your friends can come to any agreement on possible learn more about historical records of the event. Did the answers. supernova influence human history in any way? Write a two- to three-page summary of your research findings.

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MEDIA EXPLORATIONS For a complete list of media resources available, go to www.astronomyplace.com, and choose Chapter 17 from the pull-down menu.

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a Astronomy Place Web Tutorials Movies

Check out the following narrated and animated short documen- Tutorial Review of Key Concepts taries available on www.astronomyplace.com for a helpful review Use the interactive Tutorial at www.astronomyplace.com to of key ideas covered in this chapter. review key concepts from this chapter. Stellar Evolution Tutorial Lesson 1 Main-Sequence Lifetimes Lives of Stars Movie Lesson 2 Evolution of a Low-Mass Star Lesson 3 Late Stages of a High-Mass Star

Double Stars Movie

Supplementary Tutorial Exercises Use the interactive Tutorial Lessons to explore the following questions. Web Projects Stellar Evolution Tutorial, Lesson 1 Take advantage of the useful web links on www.astronomyplace. 1. Use the tool for calculating stellar lifetimes to estimate the com to assist you with the following projects. lifetimes of ten stars of different mass. Record the mass and 1. Coming Fireworks in Supernova 1987A. Astronomers believe lifetime for each of your ten stars. that the show from Supernova 1987A is not yet over. In 2. Make a graph of your results from question 1, plotting particular, sometime between now and about 2010, the mass on the x-axis and lifetime on the y-axis. expanding cloud of gas from the supernova is expected to 3. Based on your graph from question 2, briefly describe ram into surrounding material, and the heat generated in words how lifetime depends on mass for a main- by the impact is expected to create a new light show. Learn sequence star. more about how Supernova 1987A is changing and what we might expect to see from it in the future. Summarize Stellar Evolution Tutorial, Lessons 2, 3 your findings in a one- to two-page report. 1. Study the animations for both the low-mass (Lesson 2) and 2. Picturing Star Birth and Death. Photographs of stellar high-mass (Lesson 3) stars. How are the lives of low-mass birthplaces (i.e., molecular clouds) and death places (e.g., and high-mass stars similar? How are they different? planetary nebulae and supernova remnants) can be strik- 2. How do low-mass stars “move” on the H–R diagram as they ingly beautiful, but only a few such photographs are in- go through their various stages of life? cluded in this chapter. Search the Web for additional photographs of these types. Look not only for photos taken in 3. How do high-mass stars “move” on the H–R diagram as visible light, but also for photographs made from observa- they go through their various stages of life? tions in other wavelengths of light. Put each photograph r: SkyG ge a a z y e you find into a personal on-line journal, along with a one-

o r V Exploring the Sky and Solar System paragraph description of what the photograph shows. Try to compile a journal of at least 20 such photographs. Of the many activities available on the Voyager: SkyGazer CD- ROM accompanying your book, use the following ﬁles to observe key phenomena covered in this chapter. Go to the File: Demo folder for the following demonstrations: 1. Crab from Finland

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